Mast cells transfected with a TCA3-CAT construct and co-cultured with activated lymphocytes, exhibited a 5 to 7 fold increase in CAT expression which was dependent on the lymphocyte to mast cell ratio. Supernatants from activated lymphocytes had no effect. Thus, activated lymphocytes may have the ability to induce the promoter of the TCA3 gene in mast cells through a mechanism which requires cell-to-cell contact. Expression of trkA protein was demonstrated by Western blot and immunohistochemistry on HMC-1 cells. The trkA receptors expressed are functional in that nerve growth factor significantly increased mRNA levels for the early response gene c-fos. Both human and murine mast cells express lymphotactin mRNA. Expression is modulated by IL-4, TGF-beta, dexamethasone and cyclosporin A. Lymphotactin protein may be detected in mouse mast cells by an antibody against lymphotactin peptide. Human mast cells express IL-16 mRNA after activation. IL-16 is detected in human mast cells or mast cell supermatants by bioassay, ELISA, and flow cytometry. The expression of novel genes in mast cells after activation was examined with the screening of an activation cDNA library. Some of these clones isolated represented genes that are known to be upregulated in mast cells after activation, such as lymphotactin and MIP-1alpha. Three of the clones represented new genes. The expression of these genes will be examined by Northern blot analysis and the genes characterized.